1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to a system for imaging of recording media such as lithographic printing members.
2. Description of the Related Art
Imaging devices that utilize laser power sources frequently deliver the output of the laser to its destination using an optical fiber arrangement. this frees the designer from the need to physically locate the lasers directly adjacent the recording medium. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 (the entire disclosures of which are hereby incorporated by reference) disclose the use of lasers to impress images onto lithographic printing-plate constructions. As described in these patents, laser output can be generated remotely and brought to the recording blank by means of optical fibers and focusing lens assemblies.
It is important, when focusing radiation onto many types of recording medium, to maintain satisfactory depth-of-focusxe2x80x94that is, a tolerable deviation from perfect focus on the recording surface. Adequate depth-of-focus is important to construction and use of the imaging apparatus; the smaller the working depth-of-focus, the greater will be the need for fine mechanical adjustments and vulnerability to performance degradation due to the alignment shifts that can accompany normal use. Depth-of-focus is maximized by keeping output beam divergence to a minimum.
Optical efforts to reduce beam divergence also diminish power density, since a lens cannot alter the brightness of the radiation it corrects; a lens can only change the optical path.
Thus, optical correction presents an inherent tradeoff between depth-of-focus and power loss. U.S. Pat. No. 5,822,345 discloses an approach that utilizes the divergent output of a semiconductor or diode laser to optically pump a laser crystal, which itself emits laser radiation with substantially less beam divergence but comparable power density; the laser crystal converts divergent incoming radiation into a single-mode output with higher brightness. The output of the laser crystal is focused onto the surface of a recording medium to perform the imaging function.
The arrangements described in the ""345 patent employ a separate crystal for each diode pumping source. This is ordinarily necessary due to the nature of laser crystals and their operation. In the absence of optical excitation, resonant cavities formed from these optical-gain crystals are flat-flat monoliths; when optical power is delivered to an end face of such a crystal, however, this and the opposed face bowxe2x80x94an effect called bulk thermal lensing. To obtain a single transverse mode of operation (preferably the lowest-order, fundamental TEM00 mode), with the output divergence as close as possible to that of a diffraction-limited source, the crystal must be implemented in a design that accounts for bulk thermal lensing.
This phenomenon makes it even more difficult to obtain multiple, independent outputs from a single laser crystal. Even if the energy of each pumping source is confined to a discrete region on one of the crystal faces, the thermal lensing action required for lasing in one region will ordinarily affect the other regions, resulting in mutual interference. This condition is known as xe2x80x9cthermal crosstalk.xe2x80x9d Accordingly, the current state of the art prescribes the use of a separate crystal for each laser channel, resulting not only in added cost for the crystals and their mounts, but also for separate focusing assemblies.
In addition, the configurations described in the ""617 and ""092 patents (and, somewhat more pertinently, in U.S. Pat. No. 5,764,274) contemplate permanent affixation of the diode laser packages to the optical fiber. This is due to the need for efficient coupling of the laser energy into the end face of the fiber. Components are therefore permanently joined so that the alignment therebetween remains undisturbed during operation. Should a diode fail, not only the diode but the entire optical-fiber assembly must be replaced. Such a requirement is of little consequence in the arrangements described in the ""274 patent, since the the fiber is coupled to a focusing assembly using an SMA connector or the like, which is conveniently removed and replaced. In arrangements having fiber outputs that are less accessible or which require more involved mounting operations, however, permanent diode affixation at the input side of the optical fiber can prove decidedly disadvantageous.
Brief Summary of the Invention
In a first aspect, the invention confers the ability to drive a single laser crystal with multiple pumping sources to obtain discrete, collimated outputs without substantial thermal crosstalk. The meaning of the term xe2x80x9csubstantial thermal crosstalkxe2x80x9d as used herein must be understood in terms of the imaging context. Basically, it means that the action of one pumping source will not adversely interfere with the action of another source driving the same crystal; that is, an imaging output emanating from one crystal region will neither defeat nor spuriously cause an imaging output in another region. Exactly what constitutes an xe2x80x9cimaging outputxe2x80x9d also depends on the application. In a lithography environment, an xe2x80x9cimaging outputxe2x80x9d produces an image spot on the printing plate that alters the affinity of the plate for ink or a fluid to which ink will not adhere (depending on the nature of the plate). Thus, even if the laser output has some physical effect on the plate, it is not an xe2x80x9cimaging outputxe2x80x9d unless that effect translates into lithographically functional results when the plate is used. As a consequence, minor thermal crosstalk that does not rise to the level of an imaging output (or its defeat) does not qualify as xe2x80x9csubstantial thermal crosstalk.xe2x80x9d
In accordance with this aspect of the invention, measures are taken to confine the heat associated with thermal lensing to specific crystal regions, as well as to isolate these regions thermomechanically to the highest extent possible. Thus, in one embodiment, the anterior face of the laser crystal (i.e., the side facing the pumping sources) is provided with a series of parallel grooves and a pair of spaced-apart metal strips extending across the anterior face of the crystal perpendicular to the grooves. The strips and grooves serve to isolate thermomechanically the regions they define, and are aligned with the pumping sources such that the pumping-source outputs strike the anterior crystal face in the centers of the regions bounded by the strips and the grooves.
This type of configuration may involve permanent mounting of the fibers that conduct the pumping energy to the crystal. Accordingly, in a second aspect, the invention provides for removable affixation of the pumping laser diodes at the input ends of the fibers. In one embodiment, this is accomplished using a sapphire window and a mount that places the (input) end face of the fiber against the window. In another embodiment, pumping laser output is coupled into a fiber whose other end face is butted against the anterior face of the crystal.
For example, a suitable arrangement includes a laser diode; a microlens associated with the laser diode (e.g., permanently adhered to the diode output slit); a sapphire window, one side of which is associated with the microlens (e.g., permanently adhered to the lens opposite the diode slit); and a mount for removably receiving the optical fiber such that an end face thereof makes contact with the free face of the sapphire window, creating a continuous light path extending from the laser diode to the end face of the fiber. A suitable mount, adapted for an optical fiber carried in a connector comprising a threaded collar coaxially surrounding the fiber (e.g., an SMA connector), may include a tubular stem having exterior threads for receiving the collar and a bore for receiving the fiber therethrough. The sapphire window is positioned at the rear of the mount, and the relationship of elements within the mount is based on the distance the fiber protrudes into or beyond the connectorxe2x80x94ensuring that when the connector is attached, the end face of the fiber will reliably make contact with the free face of the sapphire window.
In a third aspect, the invention exploits the structure of a typical array of imaging devices to reduce imaging artifacts. This aspect of the invention pertains to any series of imaging outputs organized into one or more groups (each consisting, for example, of a multiply pumped laser crystal producing multiple outputs) and focused onto a rotating drum. Each time the drum rotates, each output of a group produces a column or xe2x80x9cswathxe2x80x9d of image points (in accordance with data corresponding to the image to be applied); the distance between adjacent swaths corresponds to the image resolution, and the outputs are indexed by this amount to begin their next swaths each time the drum finishes rotating. The invention makes use of variable indexing to disrupt the periodicity of visible artifacts associated with a particular output, thereby reducing their visual impact. Specifically, the outputs of each group of laser crystals are first indexed by the resolution distance until the regions between the adjacent output beams within each group have been fully scanned. Then, after each group is indexed by the much larger distance between the first and last outputs of a group (or, expressed with respect to a recording medium on the drum, by the axial width of the imaging zone spanned by the group of outputs), each group is again indexed by the resolution distance as before. This process is continued until all unimaged regions between neighboring groups are fully scanned.